When Is A Mare Ready To Breed?

Whether it's the manager of a breeding operation with numerous horses and a schedule to keep, or someone with a beloved mare they'd like to see pass on their genes for the next generation of equestrians in the family, the question of when a mare is ready to breed arises quite frequently. Is there an optimal age to breed a mare? Is there an age at which it is too early to breed? Are there signs that should be looked for (other than the mare cycling)?

As regards most breeds, the available literature suggests that assuming a filly is healthy and on a good plan of nutrition, she can be bred as early as two years of age, although many breeders suggest waiting until three years of age. These sources generally maintain that miniature horses should not be bred until they are four years old, since they are prone to foaling problems.¹

General Medical and Reproductive History

In evaluating a mare for breeding, it is important to know her general medical and management history plus a detailed reproductive history. A general history would include:

• age
• purchase date
• performance history
• serologic tests
• vaccination history
• boarding facilities
• feed
• previous use
• intended use
• medical history
• surgical history
• disease problems
• weight loss or gain

The mare's reproductive history should include:

• age at first heat
• heat dates
• interval between heats
• length of heats
• age first bred
• breeding dates
• foaling dates
• date of last foaling
• abnormal or assisted foalings
• number of pregnancies
• abnormal pregnancy
• previous year’s breeding cycle pattern
• number of breedings for conception
• evidence of vaginal discharge
• mothering ability
• milk production
• teasing method
• breeding method (pasture, hand breeding or artificial insemination)³

Planning, Organization and Preparation

Planning to breed a mare is a different proposition for the private horse owner (versus the manager of a breeding operation). The private owner is admonished to do ample research before deciding to breed a mare. "Not only will you need to have a thorough understanding of what is involved in breeding and foaling, but you’ll also need to know about how to choose an appropriate stallion for your mare."¹

Then, there's the expense, which may or may not fall under "research" depending on the level of knowledge the owner already possesses. As many parents who have heard the request "I want a pony!" from their five year-old can attest, horse husbandry is just not something that can be done on the cheap.

"Before you breed your mare, you need to have substantial savings. Breeding is expensive, and unfortunately, veterinary issues can occur with the mare, foal, or both. The cost of veterinary care can quickly add up, and you may run through your savings at a surprising rate. Being well-prepared financially is the first sign that you are taking the possibility of breeding your mare seriously."¹

Is the mare to be bred healthy? Moving forward, it's the single most important factor in the equation. "Your mare may seem perfectly healthy, but a breeding soundness exam by your vet is a good idea. If there's a problem, finding it well in advance of breeding season will give you time to correct it."² If the mare has foaled before, it is a good idea to have the veterinarian check for any abnormalities in the reproductive tract. Veterinarians may recommend a uterine biopsy for older mares to ensure the lining of the uterus is still in an adequate state to sustain pregnancy.

After a visual examination of the mare, the veterinarian will usually perform a rectal exam, allowing him or her to palpate the mare's ovaries, uterus and cervix.² In the late Fall and Winter, the mare won't be cycling, but the exam will give the veterinarian the opportunity to check for injuries or congenital abnormalities. Further examination of the ovaries, uterus and cervix can also be done via ultrasound.

"Some veterinarians will obtain endometrial uterine cultures through the use of a vagina speculum. An endometrial biopsy can reveal problems within the uterus that might not be detected by palpation. Hormonal analysis for progesterone, estrogen and/or testosterone may be necessary to differentiate causes of enlarged ovaries."³

In some instances, a veterinarian will also order a uterine culture and cytology, and/or a uterine biopsy. These can help to determine if the mare is suffering from a uterine infection and to detect the presence of inflammatory cells.

Pregnancy and Beyond

Of course, equine breeding operations will have established protocols for the maintenance of a mare in foal; for the first-time private breeder, determining these may require more research and preparation.

"You have to remember that being in foal and growing at the same time put additional stresses on the filly. Does she have good conformation and temperament? Does she have a healthy shape to her back or is it long and weak?"²

The private breeder will need to determine not only what plans they ultimately have for the foal, but for the appropriate accommodation of the mare and foal. Depending upon the existing accommodations available, a dedicated facility may be required for the foal. Like human parents baby-proofing their home, the owner of a mare in foal should be aware that foals require special facilities. "[T]hey need safe pastures and can get themselves trapped in small spaces that other horses cannot. You will either need to design a foal-safe facility, or will need to be prepared to send your mare away for foaling and for the initial care of the foal."²

As far as mares who have a normal working life outside of breeding, the literature suggests that mares may be ridden and lightly worked while in foal, "but should not be in hard training. Mares who are a bit older (5-6 years old), fit and working hard can be bred and kept in work until they are too heavy in foal to be comfortable working, but even then they can go for easy trail rides."¹

In planning for the breeding of a mare and determining her soundness, not all of the preparations mentioned nor all of the medical examinations will be required. Examinations will usually be predicated upon the reason for the examination, the stage of life, the stage of cycle and the status of the mare's reproductive tract. For private horse owners, the process can be as varied as an owner's resources and experience allow.

¹Harman, A., DVM. Appropriate Breeding Age. Equisearch, Aug. 2011.
²Equisearch Staff. Prepare Your Mare for Breeding. Equisearch, Nov. 2005.
³Troedsson, M.H.T., Breeding Soundness Examination of the Mare. USDA National Institute of Food and Agriculture Extension, Jan. 2020, 1-3.

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